Our long-term goal is to develop a systematic procedures for the identification of DNA damage occurring at the level of a few parts per million, or less. We have concentrated thus far upon one particular type of damage: covalent cross-links between the two chains of DNA. Cross-linking agents have been particularly effective as mutagens, carcinogens, teratogens, toxic substances, antitumor agents, antibiotics, and skin photosensitizers. In most cases, the structure of the cross-link is unknown. We have devised new methods applicable to the isolation of cross-linked nucleosides from DNA (S. Dubelman and R. Shapiro, Nucleic Acids Res. 4, 1815, 1977) and applied them to the cases of nitrous acid and formaldehyde. We now plan to determine the structure of the cross-links produced by several additional agents: mitomycin C, diepoxybutadiene, malonaldehyde, and oxidized spemine. In studying their structure, we hope to devise new microchemical tests, and improved procedures for ultraviolet and mass spectrometry. These procedures would hoepfully be applicable to structrue determination of various types of modified nucleoside, at the microgram level.